Pore development patterns and gas control effects of No.8 deep coal seam of Ordos Basin

The No.8 deep coal seam in Ordos Basin is widely distributed and spans a broad range of coal ranks, making it the primary target layer for exploration. However, a systematic and comprehensive understanding of the pore development patterns and their controlling factors in deep coal reservoirs within the basin is still lacking. In particular, there has been no in-depth research conducted on the pore development models and their gas-controlling effects in these deep coal reservoirs. Focusing on core test data from 14 exploratory wells within the basin, this study systematically examines the controlling factors and development patterns of pore structures in coal reservoirs and explores their distribution patterns and gas-controlling mechanisms. The results indicate that the pore volume of micropores and mesopores in the No.8 deep coal seam reservoirs of the study area increases significantly with the progression of coal metamorphism, while the pore volume of macropores decreases gradually. Micropores are predominantly composed of gas pores, with those in the 0.4−0.9 nm range being the most developed, whereas mesopores are relatively underdeveloped. Macropores are observed to transition from cellular pores and interclast pores in coal with low to medium metamorphic degrees to endogenetic fractures in coal with medium to high metamorphic degrees. From north to south with increasing metamorphic degree (Ro,max 0.60%−2.86%), the pore development patterns of the No.8 deep coal seam are classified into three types: macropore-dominated, dual-peak, and micropore-dominated. Macropore-dominated reservoirs exhibit lower gas content but higher permeability, favoring free gas storage and coalbed methane production. Dual-peak reservoirs represent the predominant pore development pattern, where both micropores and macropores are well-developed, creating optimal conditions for adsorbed and free gas enrichment and production. Micropore-dominated reservoirs demonstrate the strongest adsorption capacity, superior gas content, and substantial resource potential, but require extreme fracturing techniques to enhance permeability.